This proposal investigates a new mechanism of cross talk between the transcription factors Aryl hydrocarbon Receptor (AhR) and the NF-?B member RelB in the process of dendritic cell (DC) differentiation. The long-term objective of the project is to give insight into the missing knowledge about the endogenous role of the AhR and its role together with RelB in regulating gene transcription especially immune regulatory genes like cytokines and chemokines. The major impetus for this study has been provided by recent findings that RelB, a member of the NF-?B family, may play an important role in the classical AhR signaling pathway. In the present study, we will assess the physiological relevance of the AhR/RelB activity in human DC in vitro systems for the process of DC differentiation. We like to evaluate the established human myeloid cell lines U937 and THP-1 as a source of DC-like cells since most if not all studies with DC and dioxin have been performed in mice or cells derived from mouse. Further, we like to test the effect of TCDD (AhR agonist) and AhR antagonists on differentiation and the biological response of the DCs based on the measurement of selected activation markers such as surface expression of CD1a, CD40, CD80, CD86 and MHCII by flow cytometry and chemokines relevant for the function of DCs like DC-CK1, DC-STAMP, and IL-8 mRNA expression by real time PCR analysis. In project 2 of the current proposal we will identify the role of AhR and AhR/RelB activity in the differentiation process of bone marrow cells derived from C57BL/6 wild type and AhR null (AhR-/-) mice into DC. This study reveals a novel concept of the function of the AhR together with RelB and is critical in understanding how environmental toxicants like dioxins affect critical functions of the immune system and human health. PUBLIC HEALTH RELEVANCE: About 15 years ago Hankinson and coworkers identified the encoded protein ARNT, which is required for ligand-dependent translocation of the Aryl hydrocarbon Receptor (AhR) to the nucleus and its binding to Dioxin responsive elements (DRE) mediating induction of xenobiotic metabolizing enzymes (classical AhR/ARNT pathway), but the physiological role of the AhR remains a key question. The present study focuses on a new mechanism of cross talk between AhR and the NF- ?B member RelB (alternative AhR/RelB pathway), which suggests an example of how an activated AhR pathway may connect to the NF-?B subunit RelB in order to cooperatively regulate cytokine/chemokine expression as well as differentiation and function of dendritic cells. This work will help to understand the mechanism how environmental toxicants like dioxin or dioxin-like compounds, which activate the AhR, elicit immunotoxic effects and lead to adverse health effects.